1. Field of the Invention
The present invention relates to a method of cutting a workpiece with a wire saw, for example, for slicing wafers from a semiconductor silicon ingot.
2. Description of the Related art
Conventionally, there is known a method of slicing a cylindrical ingot of semiconductor material such as silicon, GaAs, etc. to form a plurality of circular disc-shaped wafers, wherein a single thin steel wire is spirally wound with tension around a plurality of parallel rollers at a predetermined pitch, and the wire is moved unidirectionally or reciprocally. When the wire is moved reciprocally, the distance over which the wire is advanced is slightly larger than the distance over which the wire is retracted, so that a new portion of the wire is pulled out every time. The ingot is pressed against the wire, while abrasive grain slurry is fed to the areas of contact between the ingot and the wire. Since this method makes it possible to cut out many wafers simultaneously, recently this method has commonly been used instead of the conventional method using an inner diameter slicer.
However, the wire becomes worn due to the abrasive grains and other causes while the wire reciprocates, and the wear amount depends on the cutting length of the workpiece. The cutting length of the workpiece is the length of the cut portion of the workpiece in contact with the wire. When the workpiece has a circular cross section, the cutting length is short at the beginning and end of slicing and is longest when the center portion of the workpiece is cut. Therefore, as shown in FIG. 5B, when the wire 1 cuts the central portion of the workpiece, the wire 1 becomes thinner compared to the case where the wire 1 cuts the upper and lower portions at the beginning and end of slicing. Accordingly, the cutting thickness (the thickness of the removed portion) changes along the cutting direction, so that the sliced wafers W come to have a nonuniform thickness, being thicker in the central part.
The wafers W of non-uniform thickness require a great amount of lapping in a subsequent lapping stage, which causes greater loss of the material of the workpiece.